FIG. 1 illustrates a schematic diagram of a prior art power factor correction (PFC) boost converter 100. An alternating-current (AC) voltage supply V.sub.AC is coupled across input terminals of a full-wave bridge rectifier BR. A first output terminal of the bridge rectifier BR is coupled to a first terminal of an inductor L. A second terminal of the inductor L is coupled to a drain of a transistor switch M and to an anode of a diode D. A cathode of the diode D is coupled to a first terminal of an output capacitor C. A second output terminal of the bridge rectifier BR is coupled to a first terminal of a sensing resistor R.sub.SENSE. A second terminal of the sensing resistor R.sub.SENSE, a source of the transistor switch M and a second terminal of the capacitor C are each coupled to a ground node. A voltage signal -I.sub.SENSE formed at the first terminal of the sensing resistor R.sub.SENSE is representative of current drawn by the boost converter 100 from the supply V.sub.AC. The signal -I.sub.SENSE is negative in polarity because it is formed by a voltage drop across the resistor R.sub.SENSE referenced to ground. A switch control voltage signal V.sub.SW which is applied to the gate of the transistor switch M determines whether the transistor switch M is conductive (switch closed) or non-conductive (switch open).
When the switch M is closed, a current flows from the bridge rectifier BR through the inductor L and through the switch M. Under such conditions, the diode D is reverse-biased by the output voltage V.sub.OUT. Current flowing through the inductor L stores energy as an electromagnetic field associated with the inductor L. When the switch M is opened, the stored energy is transferred to the output capacitor C by a current which flows through the diode D. Thus, under such conditions, the diode D is forward-biased. The energy stored in the output capacitor C form the output voltage V.sub.OUT across the capacitor C which is available for driving a load, such as a second power supply stage. A rate of energy transfer from the source V.sub.AC to the capacitor C depends upon a duty cycle of the switch control signal V.sub.SW.
An object of the boost converter 100 illustrated in FIG. 1 is to control the times at which switching of the transistor switch M occurs such that the current drawn from the alternating-current supply V.sub.AC by the boost converter 100 is substantially in phase with the voltage provided by the supply V.sub.AC and to control the duty cycle of the transistor switch M such that the output voltage V.sub.OUT is maintained at a constant level. Accordingly, the voltage V.sub.OUT and the voltage -I.sub.SENSE are both monitored for controlling switching.
The diode D is often referred to as "freewheeling" because its bias conditions change depending upon the state of the transistor switch M. For example, when the transistor switch M is closed, the freewheeling diode D is under reverse-bias conditions, while upon opening of the transistor switch M, the freewheeling diode D is under forward-bias conditions.
When such a boost converter 100 is operated in continuous conduction mode (CCM), the current flowing through the inductor L remains above zero at all times. Thus, at the instant of closure of the switch M, current is flowing through the diode D. A parasitic capacitance associated with the diode D results in a finite recovery time for the diode D such that the diode D does not turn off instantaneously. Rather, a charge stored by the parasitic capacitance of the diode D is discharged through the switch M upon its closure. A resulting high level of current in the switch M can cause excessive power dissipation and premature failure of the switch M. Because this high level of current occurs each time the switch M is cycled, the switching frequency is limited. This is especially true for boost converters which drive a second power supply stage because such boost converters typically generate a regulated voltage of approximately 400 volts across the output capacitor C. Further, because an object of the PFC boost converter 100 is to control the times at which switching occurs such that the voltage and current provided by the supply V.sub.AC are in phase with each other, this problem of elevated current in the switch M cannot conventionally be avoided by allowing the current in the diode D to fall to zero prior to closing the switch M as would occur if the converter 100 were operated discontinuous conduction mode (DCM).
Therefore, what is needed is a PFC boost converter which minimizes a current level which flows through a main transistor switch upon closure of the switch.